Degradation of polypropylene polymer to a lower molecular weight and a narrower molecular weight distribution than the starting material has been termed as being the process of visbreaking a polyolefin. The resulting polypropylene exhibits improved flowability during the fabrication of finished plastic products. Although the process of visbreaking of polypropylene can and does occur naturally at appropriate temperatures, unstabilized polypropylene oxidizing rapidly at relatively low temperature, it is common practice in the art to induce and/or to accelerate the process by means of suitable chemical reagents to obtain a product of prescribed physical characteristics.
When organic peroxides are mixed with polypropylene in the melt phase, the polymer is caused to degrade to a lower molecular weight and exhibit a higher melt flow rate, wherein the presence of the organic peroxides in the polypropylene resin results in what is known as controlled rheology (CR) resin. The peroxide of choice in the polypropylene art in the production of CR polypropylene resins is 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, available from the ATOCHEM, Organic Peroxides Division, Buffalo, N.Y., as Lupersol 101. This peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane is also the only peroxide approved by the FDA for intentional peroxide degradation of polypropylene. Although CR polypropylene resins made with Lupersol 101 exhibit good processability, the controlled rheology process of these resins under conditions of heat with Lupersol 101 causes the production of tertiary butyl alcohol (TBA) as a decomposition by product. Government regulations limit the amount of TBA present in polypropylene resins or produced therefrom to 100 parts per million (ppm).
The reduction or elimination of tertiary butyl alcohol from CR polypropylene resins containing Lupersol 101 has been the subject of continuing study in the plastics industry. Other peroxides than 2.5-dimethyl2,5-bis(t-butylperoxide)hexane are taught by Ehrig, et al., U.S. Pat. No. 4,707,524, which do not decompose to TBA and which have a half-life of from about 1.0 to 10 hrs at 128.degree. C. The peroxides of choice by Ehrig '524 are 2,2 -di(t-aryl) peroxy propane and 3,6.6.9.9 pentamethyl-3-n-propyl-1,2,4,5 tetraoxacyclononane. Ehrig '524 notes that although tests indicate that at the same concentration Lupersol 101 is more effective in controlling comparable molecular weights than the peroxides chosen by Ehrig '524, the complete elimination of TBA makes it possible to increase the concentration of the other peroxides to obtain desired physical properties of the CR polypropylene resins.
However, the peroxides taught by Ehrig '524 merely avoid the problem of elimination or reduction of production of TBA during processing of CR polypropylene resins and can introduce other problems as to modifications of process conditions and CR polypropylene resin formulas which industry-wide are based upon use of Lupersol 101.
Fodor, et al., U.S. Pat. No. 5,017,661, teaches the use of strong acidic materials, such as acidic zeolites, for reducing the level of t-butyl alcohol produced during the visbreaking of polyolefins using peroxides capable of generating t-butyl alcohol. The strong acidic materials taught by Fodor '661 include strong inorganic acids such as nitric acid, sulfuric acid, and hydrochloric acid, as well as strong organic acids such as trichloroacetic acid, trifluoroacetic acid, fluorinated alkyl sulfonic acids, and aryl sulfonic acids. Preferred however are solid strong acidic materials such as acidic zeolites. The strong acidic materials are used at a temperature within the range of from about 170.degree. C. to about 350.degree. C. at amounts of from 4:1 to about 0.25:1 weight ratio of the acidic material to the peroxide.
It is uncertain whether the process of Fodor '661 is economically desirable considering adjustments to process conditions and equipment necessitated by the utilization of strong acidic materials. It is also uncertain how effective the utilization of strong acidic materials is in reducing and controlling the melt flow-rate of visbroken polypropylene resins even though the t-butyl alcohol content is reduced. For example, in Table 3, column 4, of Fodor '661, it is observed that the melt flow (MF) of resins treated with acidic molecular sieve is not significantly less than resins not reacted with acidic molecular sieve, 37 to 39 versus 42 at the same level of Lupersol 101, although the TBA content of the visbroken polypropylene resin is significantly reduced to 28 ppm from 66 ppm.
Accordingly, in the prior art the investigators have not dealt with the problem of increasing the melt flow rate of visbroken polypropylene with use of 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane but have directed their attention to reducing the content of t-butyl alcohol in visbroken polypropylene.
It is therefore an object of this invention to provide a process for controlling and increasing the melt flow rate of visbroken polypropylene with use of 2,5-dimethyl-2,5 bis(t-butylperoxy) hexane.
It is a further object of this invention to provide a process for the production of controlled rheology polypropylene which permits and facilitates the continuous use of 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane as the peroxide of choice for visbreaking polypropylene.
It is still further an object of this invention to provide a process for the production of controlled rheology polypropylene resins wherein FDA approved thioester compounds of the general structure of ##STR2## synergistically react with the FDA-approved peroxide consisting of 2,5-dimethyl-2,5 bis(t-butylperoxy)-hexane to increase the melt flow index of controlled rheology polypropylene resins and maintain a maximum allowable level of t-butyl alcohol not greater than 100 parts per million.